The closest prior art to the present invention is believed to be EP 1426569. This patent teaches how the action of two separate cam profiles can be combined by means of a summation rocker system to achieve variable lift and event duration. It is however a disadvantage of the latter patent that in order to avoid large twisting forces it is necessary to use three cams to operate each valve. A central cam having the first profile is arranged between two other cams having the second profile. This triplication results in an expensive and bulky design, especially in an engine that uses pushrods between the cams and the rockers.